This invention relates to the construction trades, and particularly to the construction of building elements and other objects of cement by the process of spraying the cement against a receiving surface or mold, which determines what will be the surface configuration of the completed element. It is an adaptation and carrying forward of the "spray-up" process used to produce fiberglass-reinforced-plastic articles.
In the spray-up process, one or more plastic materials are fed to a gun which sprays it on a receiving surface or mold configured as the negative of an object to be constructed. The surface is initially treated with a parting compound. Then a plastic material is sprayed until a thin layer of desired thickness covers the mold. Next, short reinforcing fibers of glass for example are projected against the mold with the plastic, to act as a reinforcing medium. After spraying, the object is rolled to complete fiber embedment, cured, and removed from the mold.
Attempts to apply the spray-up technology for use with cementitious materials rather than artificial plastics have presented problems. It is the nature of a freshly mixed mortar that it contains particles or aggregates of particles which are not yet wet, and which in their then form would be too large to pass through the output orifice of the spray gun. It is also unavoidable that a certain number of fragments of hardened cement find their way into the mix, and these also are frequently too large for use. The paste of cementitious material is conducted to the spray gun by means including a progressive cavity pump comprising a special helical rotor turning in a rubber sleeve, and such a pump will pass particles of considerable size. In the gun, however, the paste issues through an aperture of limited size, and any particle in the past which is too large to pass through the aperture simply clogs the aperture, so that the gun must be disassembled and cleaned, greatly delaying and increasing the cost of the construction work.
A second major problem in adapting the "spray-up" process to cementitious materials relates to the distribution of reinforcing fibers. For use with cement, a roving of special glass strands resistant to the chemicals in mortar is available, and there has been developed a fiberglass cutter which dissevers such strands to discrete fibers of glass of chosen length and projects them by air for impingement on the wet cement. Glass fibers in short lengths are useful to reinforce plastic, but structures undertaken with cement require longer fibers for successful reinforcement, and also requires that the fibers be uniformly distributed through the subsurface material.
The above comments make it clear that the spray-up practices found satisfactory in making articles of fiberglass-reinforced-plastic are not entirely successful when objects are to be made of fiberglass-reinforced-cement. A further serious fault in the unmodified process is based on the fact that while plastic readily wets fiberglass and flows readily along and between the fibers, cement does neither readily. A superficial application of glass fibers cannot, for example, be rolled into satisfactory cohesive embedment in the body of cement.